Hands-On: Android 4.1 Jelly Bean

Depends largely on what carrier you're on. If you've got an international device, You'll probably see an update Aug/Sep. Later than the Nexus devices, but still pretty quick. AT&T/Sprint/TMo will likely have the update by the end of the year. Who knows with Verizon, I hear they're terribly slow at updating their phones.

Fortunately with the GS3, there is a LOT less that got changed per carrier, so it shouldn't take nearly as long to update as before.